Synthesizing boron nitride nanotubes (BNNTs) that are highly crystalline with few defects, and that are few-wall with aspect ratios generally exceeding 10,000 or even a 1,000,000 to 1, require stable and well-controlled self-assembly regions that are typically at high temperature. Minimizing boron and boron nitride impurities in the BNNT material produced in these synthesis processes is important for many potential uses of such BNNTs. In addition, manufacturing BNNTs in the quantities needed for many applications is an increasingly important consideration.
For the creation of BNNTs in the form of long fibers, yarns, or strings, the purity and alignment of the BNNTs is often dominated by the quality of the BNNT material in the synthesis process, as taught in International Patent Application Ser. No. PCT/US2015/27570. Current laser processes, such as described in International Patent Application Ser. No. PCT/US2015/58615, and Inductively Coupled Plasma (ICP) processes, such as described in International Patent Application Ser. No. PCT/US2014/63349, have demonstrated that they can produce BNNTs of desirable quality. However these processes may have limitations due to energy efficiency and limitations due the levels of impurities of boron particles, amorphous boron nitride (amorphous BN) particles and hexagonal boron nitride (h-BN) particles.
Generally, BNNT structures may be formed by thermally exciting a boron feedstock in a chamber in the presence of nitrogen gas. Unlike carbon nanotubes (CNTs), U.S. Pat. No. 8,206,674 to Smith et al., incorporated by reference in its entirety, indicates that BNNTs form without the presence of chemical catalysts, and preferably at elevated pressures of about 2 atm to about 250 atm. CNTs, on the other hand, typically require the presence of chemical catalysts such as metal catalysts. It has been shown that BNNTs do not form in the presence of such catalysts, indicating that the formation of BNNTs is fundamentally different than the formation of CNTs.
Most contemporary BNNT synthesis methods have severe shortcomings, including one or more of having low yield, short tubes, discontinuous production, poor crystallinity (i.e., many defects in molecular structure), and poor alignment. Additionally, many contemporary BNNT synthesis methods do not produce high quality BNNTs. Although there is no agreed upon standard in the scientific literature, the term ‘high quality’ BNNTs generally refers to long, flexible, molecules with few defects in the crystalline structure of the molecule. Apart from the Applicant's processes, there are no other reports of the synthesis of continuous BNNT fibers or BNNT strands, particularly having few defects and good alignment. The BNNT “streamers” described in U.S. Pat. No. 8,206,674 to Smith et al., for example, form near a nucleation site such as the surface of the boron feedstock, but were limited to about 1 cm in length. BNNT “streamers” at such lengths are inadequate for producing BNNT fibers and yarns.
What is needed are apparatus, systems, and methods, for the continuous production of BNNT fibers and BNNT strands, having few defects and good alignment. The Applicant has described such apparatus, systems, and methods in related applications. For example, in International Patent Application No. PCT/US15/27570 (incorporated by reference in its entirety), Applicant describes, inter alia, the continuous formation of BNNT fibers, BNNT strands, and BNNT yarns. In that disclosure, Applicant provides embodiments in which one or more lasers provide thermal excitement for generating a boron melt.
While driving the synthesis of high quality BNNT via laser driven embodiments is effective, as described in International Patent Application Ser. No. PCT/US2015/58615, the laser driven processes are relatively inefficient from the conversion of electrical energy, or other forms of energy, into the final high quality BNNT, and consequently can be difficult to scale to very high powers. For example, known laser driven BNNT synthesis systems are less than 5 kW average power.
For decades radio frequency (RF) Induction technology has been utilized to melt materials at power levels ranging from watts to megawatts. Items ranging in size from less than a finger ring to large vats of material have melted. However, RF technology has not been used for synthesizing BNNTs at high temperatures, i.e. above the melting point of boron, and in particular has not been implemented for synthesizing high quality BNNTs. When RF Induction is utilized for heating a boron melt to synthesize high quality BNNTs, RF Induction will be referred to as Direct Induction.
RF Induction heating is commonly used to heat solids for the purposes of surface modification. This can result in solid-state reactions within the solid (for example, the heat treatment process of austenitization, which may occur only at the surface or at any depth within the material) or processes in which the surface reacts with an atmosphere (carburizing, nitriding, boriding, etc.) RF Inductive heating applications such as forging or welding are not relevant here. RF Induction heating is also used extensively to melt metals for refining, alloying, and casting operations. While RF Induction heating has recently been used to process non-conductive nonmetals that become conductive at high temperatures (for example, silicon crystal growth, crystal refinement, or skull melting of cubic zirconia, etc.), in these applications all of the chemical reactions take place within the melt.
Additionally, Direct Induction has not been utilized for the synthesis or carbon nanotube (CNT) self assembly process. It should be noted that the processes and systems described herein do not apply to the formation of carbon nanotubes (CNTs). High temperature BNNT synthesis processes and systems generally involve forming a liquid material, referred to herein as a boron melt, from a boron feedstock, in more or less steady state and at very high temperature, in a nitrogen environment at an elevated pressure, such that the process produces combination of the liquid material and the gas, without involving catalysts or other elemental chemically reactive species. On the other hand, CNTs synthesis usually requires metal catalyst or other elements such as hydrogen that do not end up in the CNTs except as impurities. Certain arc discharge and laser processes will make limited quantities of CNTs, usually in vacuum, low pressure environments of hydrocarbon gases or inert gases. As a final example of the differences between the synthesis of BNNT and CNT, a CNT synthesis processes involve having a steady state ball of liquid carbon without catalysts would minimally require a temperature of 4,300° C. just to achieve the liquid carbon state and a temperature higher than this to achieve any level of CNT self assembly in a region of pure carbon gas that would have to be at a nearly equally high temperature.
Accordingly, what is needed are energy-efficient apparatus, systems, and methods, for synthesizing BNNTs, including high quality BNNTs. Further, such apparatus, systems, and methods, should be capable of synthesizing BNNTs at sufficient manufacturing quantities to enable numerous applications of BNNTs. Additionally, such apparatus, systems, and methods, should be capable of producing BNNT fibers, strings, and yarns, including highly aligned BNNT fibers, strings, and yams.